Acceleration detector

Abstract: The acceleration applied to a selected device such as a magnetic disc drive system can be detected by the use of an acceleration detector having at least one piezoelectric acceleration sensor unit. The sensor unit includes a piezoelectric element, a supporting member for the piezoelectric element and two electrical leads. The piezoelectric element is formed with a pair of elongate sheets made of a piezoelectric ceramic and an intermediate insulation layer interposed between the elongate sheets to generate a voltage difference instantaneously therebetween in response to the acceleration of the acceleration detector. The support member is made of an electrically insulating resin, and has a concave which is formed with side and bottom walls and a top opening. The side wall is provided with a slit in which the piezoelectric element is tightly caught in such a manner as to divide the concave into two open-chambers and be supported by the support member in a cantilever fashion. The electrical leads extend into the concave to make electrical connections between the elongate sheets and the electrical leads. In the present invention, since the piezoelectric element can be attached accurately at a predetermined direction to the supporting member by forcing it into the slit, it is possible to stably supply the acceleration detector having excellent sensitivity to the acceleration without causing variations in detecting direction of the acceleration.

Description: BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention relates to an acceleration detector using a piezoelectric element, which is adapted to be mounted to a selected device and capable of generating an electrical output signal when it is subjected to vibrations or shock. BACKGROUND ART Magnetic disc drive systems have been widely used as an external memory device of a computer. When a magnetic disc drive system is subjected to vibrations or shock, it is necessary to detect the acceleration of the disc drive system and to protect the magnetic disc rotated at high speed by stopping the operations of read/write heads in response to the detected acceleration. U.S. Pat. No. 5,235,472 proposes a shock sensor 1K mounted on a computer disc drive. As shown in FIG. 17, the shock sensor 1K is provided with a subassembly 10K comprised of piezoelectric polymer film 11K disposed between top metallized layers 12K and lower metallized layer 13K , and an electrically shielded case 40K for incorporating the subassembly 10K therein. The shielded case 40K is formed with a base 41K having a supporting member 20K for supporting the subassembly 10K and receiving first and second electrical leads 30K and 31K , a rectangular-shaped frame 42K providing structural integrity of the shock sensor 1K , and a metallic cover 43K providing side and top electrical shielding. The subassembly 10K is mounted to the supporting member 20K such that the top metallized layers 12K are connected to the first electrical leads 30K , and the lower metallized layer is connected to the second electrical lead 31K . The top and lower metallized layers ( 12K and 13K ) are connected to amplification and comparator circuitry (not shown) through the first and second electrical leads ( 30K and 31K ). The amplification and comparator circuitry issues a write fault signal upon detection of external force in excess of a predetermined threshold. This prior art also describes that the shock sensor 1K may be mounted on the computer disk drive in such a manner as to be a 45 degree(s) angle to each of the x, y and z axes of an orthogonal coordinate system so that linear and torsional forces imposed thereto may be monitored. For operating the shock sensor 1K stably for an extended time, it is necessary to securely attach the subassembly 10K to the supporting member 20K of the base 41K . However, since the shock sensor 1K of this prior art uses the piezoelectric polymer film 11K with relatively low thermal resistance, there is a problem of causing a breakage of the polymer film 11K when fixing the subassembly 10K to the supporting member 20K by soldering. In addition, since the subassembly 10K is mounted on the supporting member 20K at an inclined angle to the base 41K , care must be used to accurately fix the subassembly 10K to a predetermined position on the supporting member 20K by gluing. Such a care may create a difficulty of production of the shock sensor 1K . If the care is not used, shock sensors having undesired detecting directions of the acceleration would be produced with a considerable probability. As to a shock sensor to be mounted on disk drive units of portable disk-top, lap-top and notebook computers, etc., it should be also noted that the need for miniaturize the shock sensor is particularly critical as well as the need for improving the reliability of the shock sensor. In the shock sensor 1K of FIG. 17, most of a space lying around the subassembly 10K in the shield case 40K is not utilized for detecting the acceleration. Therefore, it would be desired to make the shock sensor 1K as a whole smaller by reducing the above indicated useless space without sacrifice of sensitivity of the shock sensor to the vibrations or shock. German Patent Specification DE-A-3424005 discloses an acceleration detector in which a piezoelectric element is encapsulated in a housing. Therefore, it is a concern of the present invention to provide an acceleration detector using at least one piezoelectric element which is supported by a unique supporting member to miniaturize the acceleration detector and obtain excellent sensitivity of the acceleration detector to the acceleration. In accordance with the present invention there is provided an acceleration detector as set out in claim 1. The above and the other characteristic features and advantages of the present invention will become apparent to those skilled in the art as disclosure is made in the following description of a preferred embodiment of the invention, as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a piezoelectric acceleration sensor unit and metallic shield case of an acceleration detector of a first embodiment of the present invention; FIGS. 2A and 2B are a perspective view (partially cross-sectioned) of a piezoelectric element and a perspective view of a supporting member of the first embodiment, respectively; FIG. 3 explains a voltage difference generated in the piezoelectric element when the acceleration F is applied thereto; In FIGS. 4A to 4C, FIG 4A is a top view of a supporting member integrally molded with a metal frame, FIG. 4B is a cross-sectional view of the supporting member taken along the line I-I of FIG. 4A, and FIG. 4C is a cross-sectional view of the supporting member taken along the line II-II of FIG. 4A; FIG. 5 shows a modification of the supporting member; FIG. 6 shows another modification of the supporting member; FIG. 7 shows still another modification of the supporting member; In FIGS. 8A to 8C, FIG 8A is a top view of the supporting member of FIG. 4A mounting a piezoelectric element thereon, FIG. 8B is a cross-sectional view of the supporting member taken along the line III-III of FIG. 8A, and FIG. 8C is a cross-sectional view of the supporting member taken along the line IV-IV of FIG. 8A; FIG. 9 is a top view illustrating an auxiliary groove of a supporting member; In FIGS. 10A to 10C, FIG 10A is a top (partially cross-sectioned) view of a metallic shield case enclosing the supporting member and piezoelectric element therein, FIG. 10B is an end view of the shield case of FIG. 10A, and FIG. 10C is a cross-sectional view of the shield case taken along the line V-V of FIG. 10A; In FIGS. 11A to 11D showing a modification of the shield case of FIG. 10A, FIG. 11A is a top view, FIG. 11B is a side view, and FIG. 11C is an end view; FIG. 12 is a cross-sectional view of a supporting member for the shield case of FIG. 11A; In FIGS. 13A to 13C, FIG 13A is a top (partially cross-sectioned) view of the shield case of FIG. 10A molded in an insulating resin, FIG. 13B is a cross-sectional view of the shield case taken along the line VI-VI of FIG. 13A, and FIG. 13C is a side view of the shield case of FIG. 13A; FIGS. 14A and 14B illustrate preferred shapes of earth terminals, respectively; FIG. 15 is a perspective view of an acceleration detector having two piezoelectric acceleration sensor units of a second embodiment of the present invention; FIG. 16 is a perspective view of an acceleration detector of a first modification of the second embodiment; and FIG. 17 is an exploded perspective view of a shock sensor of the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> Referring to the attached drawings, an acceleration detector 1 of the present invention is explained. The acceleration detector 1 comprises a piezoelectric acceleration sensor unit 2 and a metallic shield case 3 . The sensor unit 2 includes a piezoelectric element 10 , a supporting member 20 for supporting the piezoelectric element 10 and two electrical leads 30 , as shown in FIG. 1. The piezoelectric element 10 used in the present invention is already known as a bimorph-type piezoelectric element. That is, the piezoelectric element 10 is formed with a pair of rectangular sheets 11 made of a piezoelectric ceramic and an intermediate insulation layer 12 interposed between the rectangular sheets 11 , as shown in FIG. 2A. An insulating bond for cementing the rectangular sheets together can be used as the insulation layer 12 . A pair of acceleration detecting faces are defined on the rectangular sheets 11 at the opposed surfaces of the piezoelectric element 10 . The acceleration detecting surfaces are coated with an electrically conductive metal such as nickel or silver to form electrodes 13 thereon. For example, as shown in FIG. 3, when the acceleration F is applied to the piezoelectric element 10 in a direction perpendicular to the detecting face to cause a deflection of the piezoelectric element 10 , one of the rectangular sheets 11 is positively charged, and the other one is negatively charged. Therefore, a voltage difference is generated instantaneously between the rectangular sheets 11 in response to the acceleration of the acceleration detector 1 . The acceleration of the acceleration detector 1 can be determined in accordance with the voltage difference. The supporting member 20 is made of an electrically insulating resin, and has a rectangular concave 60 formed with a pair of side walls 61 , bottom wall 62 , top opening 63 , front and rear walls ( 64 and 65 ), as shown in FIG. 2B. The front wall 64 is provided with a slit 22 in which the piezoelectric element 10 is tightly caught in such a manner as to divide the rectangular concave 60 into two open-chambers and be supported by the support member 20 in a cantilever fashion, as shown in FIG. 1. For readily forcing the piezoelectric element 10 into the slit 22 , it is preferred that chamfered portions 24 are formed at corners of the front wall 64 . Though the piezoelectric element 10 is disposed at a right angle to the supporting member 20 in this embodiment, it is possible to dispose the piezoelectric element at an inclined angle to the supporting member 20 , if necessary. A slot 27 is formed in the rear wall 65 opposed to the slit 22 to fit one end of the piezoelectric element 10 thereto. The electrical leads 30 are integrally molded with the supporting member 20 such that one ends of the electrical leads 30 extend into the rectangular concave 60 , and the other ends of the electrical leads 30 are projected outside of the support member 20 through the side walls 61 in two directions perpendicular to the acceleration detecting faces of the piezoelectric element 10 . The electrodes 13 of the piezoelectric element 10 are electrically connected to the electrical leads 30 with the use of a conductive bond 4 such as a silver paste. When the conductive bond 4 is cast into the rectangular concave 60 to make the electrical connections, there is a probability of penetrating a small amount of the conductive bond 4 to a clearance between the piezoelectric element 10 and the bottom wall 62 or the slot 27 of the rear wall 65 . However, since a groove 28 , which is formed in the bottom wall 62 and the slot 27 , effects as a trap of the conductive bond 4 , it is possible to prevent the occurrence of a short circuit between the rectangular sheets 11 by the conductive bond 4 . As shown in FIG. 2B, the groove 28 extends over the entire length of a contact face of the piezoelectric element 10 with the bottom wall 62 and the slot 27 along the intermediate insulation layer 12 of the piezoelectric element 10 . In place of the conductive bond, it is possible to connect the electrodes 13 with the electrical leads 30 by the means of laser-welding. An electrically insulating bond 5 having a stronger bonding strength than the conductive bond 4 is filled within the rectangular concave 60 on the conductive bond 4 to more tightly fix one end of the piezoelectric element 10 to the supporting member 20 , reinforce the electrical connections and improve the strength of the supporting member 20 . Since the piezoelectric element 10 is forced into the slit 22 so as to provide a liquid-tight seal therebetween, a seepage of the insulating bond 5 from the rectangular concave 60 through the slit 22 can be stopped. Since the piezoelectric element 10 is sensitive to electrical noises, it is enclosed in the metallic shield case 3 . The shield case 3 has a rectangular cross-section thereof and is formed with an aperture 40 into which the support member 20 is forced, a pair of notches 41 located at the circumference of the aperture 42 for projecting the electrical leads 30 outside of the shield case 3 without contacting with the shield case 3 , and a pair of earth terminals 42 projecting from the shield case 3 . Since the insulating bond 5 is filled within the rectangular concave 60 such that the piezoelectric element 10 is completely surrounded with the insulating bond 5 in the rectangular concave 60 , as shown in FIG. 1, the piezoelectric element 10 can be insulated from the shield case 3 by the insulating bond 5 when the supporting member 20 is forced into the aperture 40 of the shield case 3 . Next, a process of fabricating the acceleration detector 1 is explained in detail. As shown in FIGS. 4A to 4C, a metal frame 31 having the electrical leads 30 and earth terminals 42 extending parallel to the electrical leads 30 is produced by punching a metal plate along a predetermined pattern. The supporting member 30 is then integrally molded with the electrical leads 30 . At the same time, a guide member 50 having a trench 51 is integrally molded with the metal frame 31 . The trench 51 effects as a guide for accurately forcing the piezoelectric element 10 into the slit 22 of the supporting member 20 . In this embodiment, the rectangular concave 60 has the side walls 61 perpendicular to the bottom wall 62 . As modifications of the side walls 61 , it is possible to adopt side walls 61A extending from a bottom wall 62A in such a stepwise manner that an area of the bottom wall 62A is smaller than that of a top opening 63A , as shown in FIG. 5, or side walls 61B inclined to a bottom wall 62B such that an area of the bottom wall 62B is smaller than that of a top opening 63B , as shown in FIG. 6. In addition, as shown in FIG. 7, it is preferred that each of side walls 61C is formed with a perpendicular face extending from a bottom wall 62C to an intermediate position between the bottom wall 62C and a top opening 63C and an inclined face extending from the intermediate position to the top opening 63C . Since piezoelectric acceleration sensor units of these modifications are identical in structure to that of the above embodiment except for the shapes of side walls, no duplicate explanation to common parts is deemed necessary. Like parts are designated by numerals with a suffixed letter of " A ", " B " or " C ". In any of these side walls, a total amount of conductive and insulating bonds to be cast into a rectangular concave can be economized. As shown in FIG. 8A to 8C, since the piezoelectric element 10 is forced into the slit 22 of the supporting member 20 and the trench 51 of the guide member 50 , the piezoelectric element 10 can be projected from the supporting member 20 accurately in a predetermined direction. After an optimum amount of the conductive bond 4 is cast into the rectangular concave 60 to make the electrical connections between the electrical leads 30 and the electrodes 13 of the piezoelectric element 10 , and dried, the rectangular concave 60 is filled with the insulating bond 5 , as shown in FIG. 8A. As described above, the groove 28 is formed as the trap of the conductive bond 4 in the bottom wall 62 and the slot 27 of the rectangular concave 60 . When an auxiliary groove 29 is formed in the bottom wall 62 to extend perpendicularly to the groove 28 between the electrical leads 30 , as shown in FIG. 9, the electrical connections between the electrical leads 30 and the electrodes 13 can be effectively reinforced by the insulating bond 5 in the auxiliary groove 29 . After the piezoelectric element 10 is tightly fixed to the supporting member 20 in the cantilever fashion, the guide member 50 is cut away from the metal frame 31 , as shown by the dotted lines of FIG. 8A. As shown in FIG. 10A to 10C, the acceleration sensor unit 2 is enclosed in the shield case 3 by forcing the supporting member 20 into the aperture 40 of the shield case 3 . The earth terminals 42 are connected to the shield case 3 by laser-welding. The electrical leads 30 are projected outside of the shield case 3 through the notches 41 without contacting with the shield case 3 . In FIG. 10A, the numeral " 43 " designates guide portions of the earth terminals 42 which are useful for leading the shield case 3 between the earth terminals 42 extending from the metal frame 31 . As a modification of the shield case 3 , it is preferred to use a metallic shield case 3D having a substantially cross-shaped aperture 40D , as shown in FIGS 11A to 11C. A supporting member 20D to be forced into the cross-shaped aperture 40D is shown in FIG. 12. A concave 60D of the supporting member 20D has a bottom wall 62D with a groove 28D , a top opening 63D , intermediate horizontal walls 69D mounting electrical leads 30D thereon, first side walls 67D extending from the bottom wall 62D to the horizontal walls 69D , and second side walls 68D extending from the horizontal walls 69D to the top opening 63D . A piezoelectric element 10D can be stably supported within a space surrounding with the bottom wall 62D and the first side walls 67D , as shown in FIG. 12. Since a piezoelectric acceleration sensor unit of this modification is identical in structure to that of this embodiment except for the above explained features, no duplicate explanation to common parts is deemed necessary. Like parts are designated by numerals with a suffixed letter of " D ". As shown in FIG. 13A to 13C, it is preferred that the shield case 3 enclosing the sensor unit 2 therein is molded with an insulating resin 6 so as to provide an air-tight seal between the shield case 3 and supporting member 20 and project the electrical leads 30 and the earth terminals 42 from the molded resin 6 . The metal frame 31 , which is shown by the two-dotted lines of FIG. 13A, is cut away from the electrical leads 30 and the earth terminals 42 . The earth terminals 42 are soldered to a predetermined position of a circuit board to mount the acceleration detector 1 thereon and to maintain the shield case 3 at the earth level. For stably mounting the acceleration detector 1 on the circuit board, it is preferred that free ends of the earth terminals 42 are bent in a substantially L-shape so as to give stable contact of the earth terminals 42 with the circuit board, as shown in FIG. 14A, or 14B. <Second Embodiment> An acceleration detector 1E of the second embodiment has two piezoelectric acceleration sensor units 2E . The sensor units 2E are disposed in a back to back relation so as to project piezoelectric elements 10E in the opposite directions, as shown in FIG. 15. Since each of the acceleration sensor unit 2E is identical in structure to that of the first embodiment except that the sensor units 2E have a common supporting member 20E , no duplicate explanation to common parts is deemed necessary. Like parts are designated by numerals with a suffixed letter of " E ". Three mutually perpendicular axes are defined such that x and y axes are on a horizontal plane of the supporting member 20E , as shown in FIG. 15. When the acceleration detector 1E is rotated about z axis, as shown by the arrows of FIG. 15, each of the piezoelectric elements 10E receive a rotational force to thereby generate a voltage difference between rectangular sheets 11E of the piezoelectric element 10E . The acceleration applied to the acceleration detector 1E can be determined by analyzing the voltage differences obtained from the piezoelectric elements 10E . As a first modification of the acceleration detector 1E of the second embodiment, it is possible to dispose the piezoelectric elements 10E at different angles to the supporting member 20E , respectively. For example, as shown in FIG. 16, piezoelectric elements 10F are attached to a supporting member 20F such that an angle  1 between one of the piezoelectric elements 10F and z axis is +45 degrees, and an angle  2 between the other piezoelectric element 10F and the z axis is -45 degrees. Therefore, when a pair of acceleration detecting faces are defined on opposed surfaces of the piezoelectric element 10F , the detecting faces of one of the piezoelectric elements 10F are in a perpendicular relation to those of the other piezoelectric element 10F . When the acceleration is applied to the acceleration detector of FIG. 16 in many different directions, a voltage difference V1 generated between electrical leads 30F and 32F , a voltage difference V2 generated between electrical leads 30F and 33F , and a voltage difference V3 generated between the electrical leads 32F and 33F , are listed on TABLE 1, respectively. A voltage difference between the electrical leads 30F is zero. " Vg " designates a standard voltage. Consequently, the acceleration detector of FIG. 16 is capable of detecting the acceleration instantaneously with respect to a rotational direction, X and Z axes directions, and directions perpendicular and parallel to the acceleration detecting face of the piezoelectric element 10F . Directions of the acceleration V1 V2 V3 rotation (α) +Vg/ 2 +Vg/ 2 + 2 Vg rotation (β) -Vg/ 2 -Vg/ 2 - 2 Vg X axis (+) +Vg/ 2 -Vg/ 2 0 X axis (-) -Vg/ 2 +Vg/ 2 0 Z axis (+) -Vg/ 2 -Vg/ 2 - 2 Vg Z axis (-) +Vg/ 2 +Vg/ 2 + 2 Vg 1 direction 0 -Vg -Vg 2 direction 0 +Vg +Vg perpendicular to 1 direction -Vg 0 -Vg perpendicular to 2 direction +Vg 0 +Vg As a second modification of the acceleration detector 1E of the second embodiment, it is preferred that both of two piezoelectric elements are disposed at an inclined angle to a supporting member.

Claims: An acceleration detector having at least one piezoelectric acceleration sensor unit, said sensor unit comprising: a piezoelectric element (10) formed from a pair of elongate sheets (11) made of a piezoelectric ceramic sandwiching an intermediate insulation layer (12); a support member (20) made from an electrically insulating resin in which said piezoelectric element is mounted; and a pair of electrical leads (30) contacting said elongate sheets on either side of said piezoelectric element (10), and characterised in that the support member (20) comprises a hollow body having side walls (61), a base wall (62), a front wall (64) and a rear wall (65), the walls together defining a hollow (60) having a top opening (63) and wherein said front wall has an open-ended slot (22) formed therein, the open end of the slot lying in the plane of said top-opening (63), said piezoelectric element (10) having one end thereof passed through said slot (22) and said top opening so as to divide said hollow into two open-ended chambers, the piezoelectric element (10) being a tight fit in said slot, and wherein said leads extend into respective ones of said chambers from outside the support member, one end of each lead being connected to one of said elongate sheets (11) by a conductive bond supplied via said top opening (63). An acceleration detector according to claim 1, wherein said sensor unit is enclosed in a metallic shield case (3). An acceleration detector according to claim 2, wherein said shield case has an aperture (40) into which said support member is a forced fit, and is further provided with notches (41) formed at the perimeter of said aperture for receiving said projecting electrical leads without the leads contacting said case, said case also having earth terminal means (42) projecting from an outer surface thereof for maintaining said case at the earth level. An acceleration detector according to claim 3, wherein said shield case enclosing said sensor unit therein is molded within a resin (6) so as to provide an air-tight seal between said supporting member and said shield case, said electrical leads and said earth terminal means projecting from said molded resin. An acceleration detector according to any preceding claim, wherein said electrical leads are integrally molded with said supporting member. An acceleration detector according to any preceding claim, wherein an electrically insulating bond having a stronger bonding strength than said conductive bond is filled within said hollow to tightly fix one end of said piezoelectric element to said supporting member. An acceleration detector according to claim 6, wherein a first groove (28) is formed in said base wall (62) to provide a trap for said conductive bond. An acceleration detector as claimed in claim 1, wherein a second groove (27) is formed in said rear wall (65) to receive the end of said piezoelectric element. An acceleration detector according to any preceding claim, wherein said side walls (61) extend from said base wall (62) in a stepwise manner such that the area of said base wall (62) is smaller than that of said top opening (63). An acceleration detector according to any one of claims 5 to 8, wherein said side walls are inclined such that the area of said base wall is smaller than that of said top opening (63). An acceleration detector according to any preceding claim, wherein said acceleration detector has two sensor units. An acceleration detector according to claim 11, wherein said sensor units are disposed in a back to back relation so that said piezoelectric elements project in opposite directions from their respective support units. An acceleration detector according to claim 12, wherein the supporting members of said two sensor units are integrally molded together. An acceleration detector according to claim 13, wherein said piezoelectric elements are respectively disposed at different angles to said supporting members. An acceleration detector according to claim 7, wherein an auxiliary groove (29) is formed in said base wall (62) to extend perpendicular to said first groove (28) and between said electrical leads (30). An acceleration detector according to any preceding claim, wherein said piezoelectric element (10) is disposed at a right angle to said base wall (62) of said supporting member (20).

Inventors: FUKUZONO HIDEKI, KAJI NORIMASA, KODO MASAHIRO, NISHIMURA HIROMI, TAKAMI HIROYUKI, TAKEYAMA HIDETOSHI, TANAKA HIROHISA, FUKUZONO, HIDEKI, KAJI, NORIMASA, KODO, MASAHIRO, NISHIMURA, HIROMI, TAKAMI, HIROYUKI, TAKEYAMA, HIDETOSHI, TANAKA, HIROHISA
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1994
Main classification: G01P15
IPC codes: G01P15, G01P1